How to use the Rigify building blocks for non-human rigs?
Create an armature and enter its edit mode. Delete the bone to start fresh.

You can build a custom metarig from "samples" that you can add in. Later this metarig is used to generate the rigify rig.



To know what samples to use for your use case try them out and see what they do.

Each sample will be added in the scene origin. Position the samples like you want, align them to your character. Parent them to form the metarig. You can create the human metarig that you can add as a whole from samples or you can create your own metarig (one with 4 arms for example). There are also animal parts available.

You can mix them how ever you like.

You can also add your own bones and make them 'Rigify-able'. In Pose mode select a bone and inside Rigify Type tab in bone properties you can change the parameters:



You can change on which armature layers will Rigify generate bones and customize other settings. This can be changed on the sample metarigs and on the individual samples as well.

You can see what the options do in the addon documentation (Advanced usage section).

Then generate the Rigify rig from your metarig.